The bacterium that causes stomach ulcers morphs at a breakneck pace when it first infects a person, apparently to dodge immune attack, researchers report June 13 in Nature Communications.

Helicobacter pylori sets up shop in the human stomach and can reside there for years or decades. Physician Barry Marshall of the University of Western Australia in Crawley led an international team that investigated how the bacterium evolves in a human host.

The scientists obtained samples of H. pylori from two volunteers infected with the microbe and then wiped out the infections with antibiotics. Three months later, the team reinfected both people with their own H. pylori bacteria. During the early weeks of the new infection, the bacteria mutated 40 to 50 times as fast as the earlier chronic infection samples.

The bacteria’s outer membrane proteins, which may be recognized by key players of the immune system, seemed particularly prone to change. The work suggests that H. pylori genes encoding those proteins come under intense pressure by the immune system during initial acute infection.